1. Field of the Invention
The present invention concerns franking machines with a print head removably mounted on a base unit in which the print head is a print drum carrying print wheels. It is more particularly concerned with the print wheel adjustment device in such machines.
2. Description of the Prior Art
Known print wheels are rotatably mounted on the print drum to enable their position to be adjusted when no franking operation is in progress. They project slightly from the periphery of the drum and rotate with it to print fixed and variable franking information on each rotation of the drum. The wheels print variable information: franking amount and date. The print drum therefore comprises two sets of wheel for this variable information, projecting from the drum through appropriate openings in a printing plate carrying the franking imprint. This printing plate is mated to the periphery of the drum and carries the fixed franking information in the form of the franking imprint or the imprint of the local post office or of any other kind. The same drum usually carries another printing plate for promotional devices, similar to the first but often retractable. In some machines other printing means such as one or more retractable stamps are associated with the drum.
U.S. Pat. No. 4 090 063 describes a franking machine of this type with a manually controlled device for adjusting each value print wheel. This device comprises, between each print wheel and its control system, identical transmission means with driving gears coupled to each other and to the manual control device and one or more driven gears coupled to each other and to the print wheel concerned, with two racks coupled to each other and to one of the driving and driven gears.
Other franking machines use automatic control of adjustment of the position of each print wheel rather than manual control. This automatic control is exercised from the keypad of the machine via logic circuits connected to a motor driving one of the driving gears as described above.
In removable print head franking machines the print wheel position adjustment control device and the associated motors are mounted in the print head. This makes the print head relatively bulky and heavy, with the result that it is more difficult to transport. Also, the removable print head is a secure unit to which access by the user and any other person not authorized by the postal authorities is prohibited. The authorized staff member may therefore need to be called in to deal with any maintenance problem concerning the print wheel adjustment device, which includes any complex parts.
One object of the present invention is to avoid such drawbacks.